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- Aluminium stabilizes dissolved organic matter by precipitation (2008)
- It is well known that carbon mineralization in acidic forest soils can be reduced by large Al concentrations. Precipitation of dissolved organic matter (DOM) by dissolved Al could contribute substantially to C retention in acidic forest soils. However, no information is available on the properties of precipitated organic matter (OM) and its stability against microbial decay although that might be an important mechanism for long-term carbon storage in soils. Therefore, I investigated the influence of (i) the DOM composition, (ii) the pH of precipitation, and (iii) the Al/C ratio in solution on the amount of OM precipitated, the associated changes in its composition, and the resulting stability of precipitated OM against microbial decay. Between 13 and 84% of the C in solution was precipitated, depending on pH, Al/C ratio, and the composition of DOM. I observed preferential precipitation of aromatic compounds and enrichment of carboxylic C in precipitated OM, being associated by depletion in N and enrichment in P. The type of bonds between OM functional groups and the Al cations was independent of pH, Al/C ratios and solution composition. FTIR spectra indicated ligand exchange as possible binding mechanism. Furthermore, exoenzymes were present in a functional state in precipitated OM. Carbon mineralization of DOM was up to 28 times larger than that of the respective precipitated OM. Only 0.5-7.7% of precipitated C was mineralized during 7 weeks of incubation. When precipitated OM remained in solution the reduction in C degradation by precipitation amounted up to 65%. This increase in stability by precipitation was significantly correlated with the amount of C precipitated. Thus, the bonds between Al and carboxylic groups of aromatic compounds with low N contents resulted in a reduced bioavailability of C. I found no indication for toxic effects of Al. The enzyme activity found in precipitates significantly increased the percentage of C mineralized of precipitated OM. However, after eight weeks of incubation the correlations between enzyme activity and C mineralization disappeared, despite substantial enzyme activity and C being still present. Thus, degradation of precipitated OM seems to be governed by enzyme activity during the first degradation phase, but the long term stability of precipitated OM is probably related to its chemical properties and structure, e.g. floc size. The Al cations linked a large number of molecules of OM up to floc sizes of 110 µm in diameter, with smaller sizes on average at pH 3.8 (16.6 µm) than at pH 4.5 (27.6 µm). For a floc of 10 µm about 80 million molecules needed to be spatially aggregated. Thus, the spatial accessibility of OM was considerably reduced in these flocs leading to additional stabilization, besides the intrinsic stability of the organic compounds and the complexation by Al. The observation that only a certain fraction of precipitated OM is prone to degradation by enzymes supports this. I conclude that precipitation of DOM is an important mechanism for the long-term carbon stabilization in mineral horizons of acidic forest soils. Changing environmental conditions can affect both the amount of OM precipitated and the degradability of the precipitate. Thus, more focus should be laid on the influence of OM precipitation on C retention and sequestration in acidic forest soils.